Surveys of Calling Amphibians in North Dakota

Results and Discussion

Eight of the nine anurans known to occupy North Dakota were encountered in the
survey: plains spadefoot toad (Scaphiopus bombifrons), Canadian toad
(Bufo hemiophrys), Woodhouse's toad (B. woodhousii), Great Plains
toad (B. cognatus), gray treefrog (Hyla versicolor), western chorus
frog (Pseudacris triseriata), northern leopard frog (Rana pipiens),
and wood frog (R. sylvatica). We did not record the American toad (Bufo
americanus). Locations where each species was detected relative to the range
of each as described by Hoberg and Gause (1992) are given in Figs. 1-8.

For three species, we had detections outside the Hoberg-Gause ranges. The
Plains spadefoot toad was detected in Towner and Wells counties, both somewhat
east of the limit suggested by Hoberg and Gause. We did not find it in the
western half of the state, which is within the Hoberg-Gause range. Bowers
et al. (1997) also found Plains spadefoot toads east of the indicated range.
We found Woodhouse's toad in Bottineau, Eddy, and Cass counties; the species
was present at two hexagons in the last-named county. These counties are north
and east of the range indicated by Hoberg and Gause. We recorded the Great
Plains toad in two of the northernmost counties, Bottineau and Towner, just
north of the range indicated by Hoberg and Gause.

Conversely, we found Canadian toads only in the northeastern portion of
the published range, and the gray treefrog only in the northernmost county
of the North Dakota range. The northern leopard frog, although having a statewide
distribution, was detected only in the southeastern quadrant of the state.
Much of North Dakota had been under drought conditions for a number of years,
ending in 1993. Conceivably, certain species had not rebounded from those
adverse conditions by 1995, thus resulting in reduced abundance and range.

For analyses of features that might influence the detection of calling amphibians,
we concentrate on the five species that were recorded on more than 10 stations
(Table 2), but mention conditions under which the
three uncommon species were recorded.

Date.--Plains spadefoot toads were documented from late May through
mid June. We recorded the Canadian toad in both early and late June (Fig
9). Woodhouse's toads, Great Plains toads, and northern leopard frogs
were recorded more frequently than expected in late May through early June.
Bowers et al. (1997) showed calling activity of Woodhouse's toads varied irregularly
from mid May until the beginning of July. They detected Great Plains toads
most commonly throughout June. They found northern leopard frogs in the first
three weeks of May and late June-early July. In Wisconsin, northern leopard
frogs were recorded more frequently in late April surveys than in surveys
in late May or later (Mossman and Hine 1984). Bishop et al. (1996) indicated
that the peak period for northern leopard frogs in Ontario was from 1 April
or earlier to 31 May. The chorus frog was more common than expected throughout
May, and less common in June. Bowers et al. (1997) detected them throughout
their surveys but peaking prior to mid June. Chorus frogs were more frequent
in Wisconsin during the early survey period (15 - 30 April; Mossman and Hine
1984). The only occurrence of wood frogs in our survey was on 22 May; those
of the gray treefrog were on 26 June. Bowers et al. (1997) found wood frogs
only in early May, and gray treefrogs in mid June and early July. In Wisconsin,
wood frogs were detected in April surveys (Mossman and Hine 1984), and treefrogs
were noted from late May through June. Bishop et al. (1996) listed peak periods
as 1 April - 17 May for the wood frog and 9 May - 8 July for the tetraploid
gray treefrog.

Time of night.--We noted plains spadefoot toads between 2330 and
0200. Canadian toads had a clear peak at about 2400 hours (Fig.
10). Woodhouse's toads in our surveys were heard more frequently from
about 2300 to 0130 hours, after which calling diminished; results of Bowers
et al. (1997) were similar. Great Plains toads followed a similar pattern,
except beginning slightly earlier; Bowers et al. (1997) detected them regularly
between 2130 and 0230 hours. The only gray treefrogs were heard at 2400 hours
or just before. Conversely, Bowers et al. (1997) detected them mostly 0100
and 0200 hours. Chorus frogs called independently of time of night; observations
closely matched time of survey. Bowers et al. (1997) recorded greater numbers
between 2030 and 2200 hours, although they found them during all time periods.
Northern leopard frogs were detected somewhat more frequently than expected
from 0030 to 0130, after which time they were not detected. Bowers et al.
(1997) noted them mostly during 2130-2200 hours and 0030-0100 hours. Wood
frogs were noted only at 0130 hours.

Temperature.--No relation to temperature was noted for the plains
spadefoot toad. The only departure from expected for the Canadian toad was
that it seemed most active at temperatures near 70°F (Fig.
11). Woodhouse's toads also seemed more active at temperatures of about
65-70°F. Bowers et al. (1997) found no favored temperature range for Woodhouse's
toads. Great Plains toads called more than expected when temperatures were
60°F or above, and less than expected at lower temperatures. In contrast,
Bowers et al. (1997) indicated that they called actively across the full range
of temperatures; their Fig. 5 shows a peak between 50 and 77°F, however.
The gray treefrog was recorded when temperatures were near 65°F. Bowers
et al. (1997) indicated that treefrogs called more frequently at temperatures
between 59 and 77°F. Shirose et al. (in press) noted that a hot, dry period
in June interrupted the breeding activity of gray treefrogs. We recorded the
chorus frog slightly more frequently when temperatures were below 55°F,
but it was heard at all temperatures encountered; Bowers et al. (1997), in
contrast, suggested that activity declined uniformly with increasing temperatures.
Northern leopard frogs were more commonly detected at temperatures of about
60 to 70°F. Bowers et al. (1997) found them more frequently when temperatures
were between 41 and 59°F. The wood frog was heard when the temperature
was only 40°F; Bowers et al. (1997) also noted more calling at lower temperatures
(41 - 59°F).

Humidity.--No effect of humidity was apparent for the plains spadefoot
toad, Woodhouse's toad, Great Plains toad, chorus frog, or northern leopard
frog (Fig. 12). The Canadian toad was more frequently
heard than expected when the relative humidity was about 90 percent, and the
gray treefrog and wood frog were encountered only when the humidity was 90
percent. Bowers et al. (1997) indicated that Woodhouse's toads and northern
leopard frogs called more frequently when humidity was below 65 percent, whereas
the other four species' calling activities peaked between 55 and 90 percent
humidity.

Precipitation.--Precipitation seemed not to influence markedly detections
of any species (Fig. 13), but calling was slightly
more frequent than expected when rain was absent for all species except the
rarely heard plains spadefoot toad, for which the reverse was true. Bishop
et al. (1996) had some limited evidence that gray treefrogs were detected
more often during rain.

Wind speed.--The plains spadefoot toad was recorded mostly under
calm conditions, but was detected once when the Beaufort value was 4. Calling
of the Canadian toad and Woodhouse's toad were not clearly correlated with
winds (Fig. 14). The Great Plains toad was detected
more frequently when winds were still, and less frequently when the Beaufort
scale reading exceeded 1. The gray treefrog was detected only at a Beaufort
reading of 1. The chorus frog was generally not influenced by wind speed,
except for a minor dropoff at Beaufort 4; Bishop et al. (1996) detected a
similar pattern for spring peepers (Pseudacris crucifer). The northern
leopard frog and wood frog were recorded only under still conditions.

Cloud cover.--The plains spadefoot toad and northern leopard frog
were recorded somewhat more often under moderately cloudy conditions. Canadian
toad calling was more frequent under heavy cloud cover (Fig.
15). Woodhouse's toads and Great Plains toads showed a reverse pattern,
being detected somewhat more often when cloud cover was less than 50 percent.
The two detections of gray treefrogs occurred under cloudy conditions. The
chorus frog was unaffected by cloud cover. The only detection of wood frog
was under conditions of medium cloud cover.

Moonlight.--Canadian toads, Great Plains toads, northern leopard
frogs, and possibly plains spadefoot toads were noted more often than expected
under darker conditions (Fig. 16). The gray treefrog
and wood frog were detected only under the darkest conditions. Chorus frogs
and Woodhouse's toads seemed unaffected by moonlight.